“The problem with previous methods of single-cell injection was low cell viability and low efficacy,” Ingrid Wilke, assistant professor of physics at Rensselaer Polytechnic Institute said. The researchers used tightly focused femtosecond laser beam pulses that created a pore or opening in the cellular wall of living cells and encouraged the cell to take in different molecules.

The laser beam serves as a “needle” that punctures the protective skin around the cell, encouraging the cell to take up the material surrounding it. In this case, the researchers used a yellow iodine dye as their nanoscale “vaccine” so the injection results could be easily viewed in microscopic images.

A femtosecond is one billionth of one millionth of a second. The lasers emit radiation in the near-infrared (NIR) portion of the spectrum, meaning that the wavelength is too long to be seen by human eyes. Upon analysis, the femtosecond NIR lasers were found to preserve the integrity of the cells, Wilke said. But only up to a certain intensity.

“The connections between laser intensity and the rate of injection had not been previously explored in-depth,” Wilke said. “We found that the size of the pores was highly dependent on the intensity of the laser. By modifying the strength of the laser, we could encourage the cell to uptake as little or as much of the materials as we desired. We also determined the intensity at which the cell could first be permeated and the level at which it would be disintegrated.”

They discovered that cells were permeated at laser intensities of four terawatts per square centimetre. The pore size grew larger as the intensity increased. When the intensity reached more than 35 terawatts per square centimetre, the cellular structure disintegrated and the cell was no longer viable.